The present invention is directed to a method and apparatus for placing and electrically connecting components on a printed circuit board and more specifically to an automatic system for feeding and locating individual components relative to a printed circuit board wherein the components and circuit board have contact means which are pre-tinned or otherwise supplied in advance with solder material and which are to be connected by a reflow soldering technique utilizing a laser beam or other heating material under the control of an infrared detector which also serves to inspect the individually formed solder joints.
At the present time the manufacture of electronic printed circuit boards requires a large number of individual steps each of which is carried out by seperate pieces of equipment which requires not only a very large capital investment but which also is relatively slow due to the need for transferring the circuit board from one machine to another for each operation to be performed in sequence.
The use of pick and place machines in the production of printed circuit board assemblies is old and well known in the art. Such machines are usually controlled by a group of integrated microcomputers and provide for the automatic feeding of electronic components to a printed circuit board. One or more pick and place heads are provided which are automatically selected depending upon the type of component to be placed. Each pick and place head is usually provided with vacuum and/or a tweezer jaw assembly for picking up the specific electronic component and transferring it to the desired predetermined position on a printed circuit board. The pick and place head and/or the printed circuit board are suitably indexed in an X-Y pattern to sequentially locate all of the components in the proper position on a single circuit board. Such machines are provided with an independent adhesive applicator head which dispenses a predetermined amount of epoxy adhesive or solder paste at predetermined locations on the circuit board. Thus upon placement of the individual electronic components on the circuit board they will be held in the desired position by the epoxy adhesive or the solder paste. The completed printed circuit board assembly will then be transferred to a separate machine for soldering the components to the printed circuit board to provide the desired electrical connections therebetween. Examples of such well known pick and place devices are the MPS-500 and MPS-100 pick and place systems produced by the Dyna/Pert Division of the Emhart Machinery Group in Beverly, Massachusetts. Such pick and place systems are designed to handle small chip resistors, capacitors and diodes; small outline transistors; small outline integrated circuits and leaded or leadless chip carriers, both ceramic and plastic.
With the various components temporarily held in place on a printed circuit board by means of an adhesive the printed circuit board is then transferred to the soldering station to permanently secure the components to the printed circuit board by means of electrically conductive solder joints. The process of applying solder to form a permanent mechanical and electrical bond between two conductors on a printed circuit board is carried out in various ways which will be familiar to practitioners of the soldering art and need not be discussed at length herein. For small scale production, solder joints are individually hand soldered and for large scale production an entire circuit board requiring hundreds or thousands of solder joints can be soldered in one step by wave soldering or by reflow soldering.
According to the wave soldering method, after certain preparatory steps, the board is passed over the surface of a molten solder wave where the solder is caused to adhere to local areas at the intended joints. Such a system however is difficult to use for a printed circuit board having surface mounted leadless components.
In reflow soldering, individual solder pads are pretinned at the desired locations by use of molten solder which is then allowed to solidify. The desired electrical conductors, also pre-tinned, are then placed in mechanical contact with their proper pads and the entire board is raised to the desired temperature either by radiant heating or by various other methods. A particular problem with the reflow process is that all solder joints on a given board do not always require the same amount of solder or have different amounts of adjoining metal in contact with the solder. The result is that various solder joints will have different heat-input requirements so that standard radiant or convective heat-input methods will tend to overheat the smaller joints while underheating the larger joints. During reflow soldering it is important to know the exact moment when the solid solder turns to liquid or vice versa and it is advantageous to be able to make this determination without making physical contact with the solder joint and without having to know the radiant emissivity of the surface. Since the prior art reflow processes all involve the exposure of the entire circuit board assembly to elevated temperatures there is always the danger of damaging the electronic components during the reflow soldering process.
A new and improved reflow soldering technique has been disclosed in U.S. patent application Ser. No. 619,438, filed June 11, 1984 which is assigned to the same assignee as the present application. Since various solder joints have different heat-input requirements and since there is a danger of damaging the electronic components during a overall heating operation, application Ser. No. 619,438 discloses a method for individually heating each solder joint by means of a focused laser beam while the surface of the material being heated to its melting point is monitored by an infrared detection system so as to interrupt the application of heat upon the completion of reflow. Thus each individual joint may be soldered under controlled heating conditions without fear of damage to the associated electronic component.
In order to detect and correct various defects which might occur during the assembling and soldering of the printed circuit board, various inspection techniques are well known in the art. A visual inspection will detect the more obvious defects but such visual inspection is tedious and costly. However the principal disadvantage with visual inspection resides in the fact that humans are subject to fatigue, errors in judgement and to variability in their "accept/reject" thresholds. An assembled printed circuit board can also be tested by means of contact devices which measure electrical parameters. However, such electrical parameters usually result from the compound action of several components whose individual performance therefore cannot be checked. As a result about three percent of all printed circuits which pass conventional testing actually contain incipient defects which might cause early failure in the field.
In order to provide a more thorough inspection of assembled printed circuit boards an infrared non-contact system was devised for inspecting infrared emitting components in an electronic device. Such a system is discussed in U.S. Pat. No. 3,803,413, granted Apr. 9, 1974. While some devices radiate thermal energy as a result of electrical energization other devices require the injection of thermal energy into the component such as a solder joint to heat the component so that the infrared radiation given off can then be detected. U.S. Pat. No. 3,803,413 teaches the use of laser devices for injecting the thermal energy into the various components and a scanning system which enables the thermal energy to be injected in sequence into a specific array of components as would be found with the numerous solder joints on a printed circuit board. An improved laser-infrared inspection system is disclosed in U.S. Pat. No. 4,481,418. In this system, an infrared laser provides a thermal pulse and a visible wavelenght of another laser is used for targeting. The laser beams are made coaxial within a flexible optical fiber which serves as a beam homogenizer and which provides mechanical isolation between the lasers and the rapid movements of the positioning table. The infrared laser beam is injected into each respective solder joint and the heating of each solder joint causes the emission of infrared thermal energy from the joint which will provide a thermal signature for each specific solder joint. This signature will consist of a rise during the heat injection and a decay during the cooldown phase after the end of the heating laser pulse.
A computer is utilized for controlling the inspection and for evaluating the thermal data. The X-Y target coordinates on the positioning table are stored in a computer prior to the inspection of each design of a printed circuit board. The computer positions each target in turn on the optical axis, controls the laser's shutter operation and samples and prints out the detector signal at its peak value and three times afterwards, typically, at 5 ms intervals. It also monitors the detector output for any unusual heating conditions which might result if, due to mistargeting, a component or substrate is in danger of being burned. Standard data is provided in the computer memory against which the test data is compared by the computer. The computer is provided with means for printing out the thermal data for an entire circuit board and for flagging or identifying defects. The flagging process in which the computer identifies the defects makes use of the standard deviation values which were computed. Alternatively high and low threshold values for each of the solder joints can be entered through the keyboard based on operator judgement and experience. The circuit board is then transferred to an automatic rework station where each improper solder joint is automatically displayed to an operator who can quickly make the necessary corrections.
A problem is inherent in the resoldering of solder joints. During the soldering process intermetallic compounds are formed in every solder joint. The amount of such intermetallic compounds increases with every touchup of the solder joint causing the solder joint to become very brittle and susceptible to breakage. Therefore the correction procedures cause a condition which might lead to future failures in the solder joints, especially if the printed circuit board is subjected to various stresses during use. Therefore, it is very important to limit the amount of touchup to a bare minimum, necessary only for those solder joints that have been verified as being in real need of repair, and avoiding any routine touchup for cosmetic appearance only. The present invention allows this goal to be precisely met.